Combined microautoradiographic and histopathologic analysis of the fate of challenge Schistosoma mansoni schistosomula in mice immunized with irradiated cercariae.

Combined microautoradiographic and histopathologic methods were used to locate and examine schistosomula of Schistosoma mansoni in the lungs of irradiated cercaria-immunized mice 21 days after percutaneous challenge infection with 75Se-labeled cercariae. Of 75 schistosomula examined in serial sections, 53% were located in the pulmonary microvasculature, 23% in alveolar spaces, 3% with one end in a vessel and the other in an alveolar space, and the locations of 21% were not identified. Inflammatory reactions of variable intensity were observed around schistosomula in both vascular and alveolar sites, although the most intense category of reactions was associated almost entirely with alveolar larvae. All autoradiographic foci contained recognizable schistosomula. Although the concentration of reduced silver grains precluded cyto-structural analysis, observations on schistosomular contour and shape provided no evidence of larval damage. Our findings suggest that immune elimination of schistosomula in mice immunized with irradiated cercariae is partly or largely effected by a process of alveolar extrusion of viable parasites during their lung migration.